TweetA)It's nowt much to do with reducing side effects since there are drugs to prevent that. It has more to do with providing your body with a steady stream of anabolic compounds to produce a muscle building environment.
B)For me injection rotation has more to do with limiting scar tissue build up more than anything else. IMO, the reason site injection works with shoulders and arms in males at least is because we have more AR there for the hormones to occupy.